NMR spectroscopy of minerals and allied materials

Sharon E. Ashbrook and Daniel M. Dawson DOI: 10.1039/9781782624103-00001

Nuclear Magnetic Resonance (NMR) spectroscopy has played an important role over many years in understanding the structure and reactivity of minerals. The advent of high-resolution NMR techniques, higher magnetic field strengths and recent improvements in theoretical calculations have widened the potential use and application of NMR in mineralogy and solid-state chemistry. Here we review work from the period 2010–2014, focussing primarily on materials formally classified as minerals, but mentioning allied materials that are wholly or partly synthetic, where significant structural or mineralogical insight has been demonstrated.

1 Introduction

There are over 4600 formally recognised types of mineral, i.e., elements or chemical compounds that occur naturally as a result of geological processes. Minerals are usually defined as naturally-occurring, stable solids with a specific chemical composition (within some defined limits) and exhibiting an ordered atomic structure. In the past, minerals were typically considered to be inorganic and abiogenic, with biological substances, e.g., bones and shells, excluded from classification, although this latter point has always been the subject of some debate. However, today, many classification schemes include all biominerals, and a specific class of organic minerals is also recognised. The distinction between minerals and rocks, however, should be noted – the latter typically being aggregates containing one or more minerals and exhibiting structural and chemical heterogeneity."

The study of minerals has long been recognised to be of considerable importance – not only for understanding the fundamental physical and chemical properties of the materials that make up the surface and inner depths of our planet, but to understand the effects of variations in pressure or temperature upon these properties, and the changes that can occur due to weathering or chemical processes. Many minerals also find industrial use in, e.g., ceramics, cements, fertilisers, catalysts and glasses, making an understanding of their structure, composition and reactivity vital. A large number of materials are structurally and/or chemically related to minerals, and can be produced either by chemical modification/substitution of a mineral or by an entirely synthetic approach. While not strictly minerals (as they are not naturally formed), they nonetheless provide additional possibilities for application, and their study may well also provide insight into that of the parent/related mineral.

All of the 90 natural elements have some geochemical interest, but the bulk (~90%) of the Earth's crust is composed of silicate and alumino-silicate minerals, with elements such as Fe, Ca, Na, K and Mg also of importance, as shown in Fig. 1a. The inner regions of the Earth, i.e., where pressures and temperatures increase, are also typically composed of silicate minerals, but with increased Mg and Fe content, as shown in Fig. 1b. Figure 1 also shows the changes in the major mineral component of the Earth with increasing depth results in the designation of 'layers', e.g., the change from a-(Mg,Fe) 2SiO4 to ß-(Mg,Fe)2SiO4 at ~410 km, signifying the boundary between the upper mantle and the upper transition zone, with further transitions to ?-(Mg,Fe) 2 SiO4 at the boundary with the lower transition zone, and to (Mg,Fe)SiO3 perovskite in the lower mantle.

The requirement for an ordered atomic structure in a mineral has resulted in much previous mineralogical study being carried out using crystallographic diffraction. However, many minerals form extensive solid solutions (i.e., they exhibit a variation in chemical composition) where the exact ordering/position of substitution is not known. Diffraction provides information on the average structure, but is rarely able to provide the atomic-level detail required to understand how and why the structure and/or properties of a mineral vary with composition. This is particularly true where the difference in scattering factors is small (e.g., Al3+ and Si4+), concentrations are low, or dynamics play a significant role. The sensitivity of NMR spectroscopy to the local structure, through the variation of interactions such as the chemical shielding, J-coupling or quadrupolar coupling, provides an ideal tool for structural investigation of minerals, and the recent developments in hardware and software, enabling high-resolution NMR spectra of solids to be acquired with good sensitivity, have considerably widened the application of this technique. Despite these advances, complex spectral lineshapes can be observed for disordered materials. However, over the last 10 years, the approach of combining experiment with theoretical calculations of NMR parameters (typically using density functional theory, DFT) has grown to enable the assignment of spectral resonances and the prediction of spectra for many possible models when a structure is less well defined.

In this chapter we review the NMR spectroscopy of minerals published in the period 2010–2014. We assume a basic working knowledge of the methods used to obtain high-resolution NMR spectra of solids (e.g., MAS, decoupling, MQMAS, etc.,) and some knowledge of prior significant work on minerals, e.g., the use of 29Si NMR to study Si/Al ordering in aluminosilicates. More complete reviews on these can be found in ref. 2, 3, 5 and 6. We shall initially concern ourselves with silicate minerals, dividing these according to their structural features, e.g., materials containing isolated units, chains, layers or frameworks of silicate tetrahedra, before turning to non-silicate minerals (which we shall categorise according to their chemical type). Although we shall focus primarily on materials in the more formal classification of minerals described above, we shall also mention wholly or partly synthetic allied materials, where significant mineralogical insight has been shown.

2 Silicate minerals

As Si and O dominate the Earth's crust and much of the mantle, silicates are the most important class of rock-forming minerals, and exhibit great structural variation owing to the stability of Si–O bonds. Most crustal silicates are based on SiO44- tetrahedra, which may occur in isolation or combine to form more complicated structures. Although...